The present invention relates generally to a manner by which to select antenna-weightings at a sending station that utilizes transmit diversity. More particularly, the present invention relates to apparatus, and an associated method, by which to utilize a perturbation gradient approximation technique in the selection of the antenna weightings. Improved antenna-weighting selection accuracy is provided as a long-term covariance matrix of a channel upon which a communication signal is sent is tracked, instead of merely tracking a short-term fading process. And, the amount of feedback needed in the effectuation of the antenna-weight selection is independent of the number of transmit antennas utilized by the sending station.
Communication of information is an endemic necessity of modem society. Communication of information is effectuated through operation of a communication system. Information is communicated between a sending station and a receiving station by way of a communication channel. The sending station, if necessary, converts the information into a form to permit its communication upon the communication channels. And, the receiving station, if necessary, operates upon detected indications of the information to permit operations to be performed thereupon to recover the information.
A wide variety of different types of communication systems have been developed and are regularly utilized to effectuate communication of information between sending and receiving stations. New types of communication systems have been, and continue to be, developed and constructed as a result of advancements in communication technologies.
An exemplary communication system is a radio communication system. In a radio communication system, the communication channel is defined upon a radio link extending between the sending and receiving stations. Communication systems implemented as radio communication systems are sometimes of reduced costs, relative to wire line counterparts. And, radio communication systems are amenable to implementation as mobile communication systems since radio links, rather than fixed, wire line connections, are utilized upon which to define communication channels used to communicate the information.
A cellular communication system is exemplary of a radio communication system that has achieved significant levels of usage. Cellular communication systems have been installed throughout significant parts of the populated portions of the world. Various cellular communication standards have been promulgated, setting forth the operational parameters of different types of cellular communication systems.
Generally, a cellular communication system includes a fixed network infrastructure that includes a plurality of fixed-site base transceiver stations. The fixed-site base transceiver stations are positioned at spaced-apart locations throughout a geographical area that is to be encompassed by the communication system. Each of the base transceiver stations defines an area, referred to as a cell, from which the cellular communication system derives its name.
The fixed network infrastructure of which the base transceiver stations form portions is coupled to a core network, such as a packet data backbone or public-switched, telephonic network. Communication devices, such as computer servers, telephonic stations, etc. are, in turn, coupled to the core network, or elsewhere, and are capable of communication by way of the network infrastructure and the core network.
Portable transceivers, referred to as mobile stations, communicate with the base stations by way of radio links forming portions of the electromagnetic spectrum. Use of the cellular communication system is permitted, typically, pursuant to a service subscription, and users, referred to as subscribers, communicate by way of the cellular communication system through utilization of the mobile stations.
Information communicated upon a radio link is susceptible to distortion as a result of non-ideal communication conditions. Other communication systems are analogously non-ideal, and communication of information in such other communication systems analogously also is susceptible to distortion. The distortion causes values of the information delivered to a receiving station to differ with the corresponding values of the information, when transmitted by the sending station. If the distortion is significant, the informational content of the information cannot be accurately recovered at the receiving station.
Fading caused by multi-path transmission distorts information communicated upon a communication channel. If the communication channel exhibits significant levels of fading, the informational content of the information might not be able to be recovered.
Various techniques are utilized to compensate for, or otherwise overcome, the distortion introduced upon the information during its communication upon a communication channel to the receiving station. Space diversity, for instance, is sometimes utilized. Space diversity is created through the use, at a sending station, of more than one transmit antenna transducer from which information is transmitted. Spacial redundancy is provided therefrom. The antenna transducers are typically separated by distances great enough to ensure that the information communicated by respective antenna transducers fades in an uncorrelated manner. And, receiving stations sometimes also utilize more than one receive antenna transducer, also typically separated by appropriate separation distances.
Communication systems that utilize both multiple transmitting antenna transducers and multiple receive antenna transducers are sometimes referred to as being MIMO (Multiple-Input, Multiple-Output) systems. Communications in an MIMO system provide the possibility that higher overall capacity of the system, relative to conventional systems can be achieved. Increased number of users are able to be serviced, or more data throughput is capable of being provided for each user.
The advantages provided through the use of space diversity are further enhanced if the sending station is provided with information about the state of the interface, interfacing the sending and receiving stations, i.e., the communication channel.
A sending station is not able to measure channel characteristics of the communication channel directly. Such measurements are possible only at a receiving station. In two-way communication systems, measurements made at the receiving station can be returned to the sending station to provide an indication to the sending station of the channel characteristics.
Communication systems that provide this type of information to a multiple-antenna sending station are referred to as being systems that provide closed loop transmit diversity. Communication channels extending from the network infrastructure of a cellular communication system to a mobile station are sometimes referred to as being down link, or forward link, channels. And, channels extending from the mobile station back to the network infrastructure are sometimes referred to as being uplink, or reverse link, channels.
The feedback information returned to the sending station, here at the network infrastructure, from the receiving station, here a mobile station, is used to select values of antenna weightings. The weightings are weighting values by which information signals provided to separate ones of the antenna transducers are weighted prior to their communication upon a communication channel to the mobile station. A goal is to weight the information signals applied to the antenna transducers in manners best to facilitate communication of the information to the receiving station. The values of the antennas weightings approach a conjugate of the subspace spanned by down-link channel covariance matrix. Estimation of the antenna weightings can be formulated as a transmission subspace tracking procedure. Several closed loop transmit diversity procedures are utilized. TxAA, Eigenbeam Former, and other techniques are sometimes utilized. Existing techniques, however, suffer from various deficiencies. For instance, a TxAA procedure fails to take into account a long-term covariance matrix of the communication channel in the selection of the antenna weightings. And, use of an Eigenbeam former technique is dependent upon the number of antenna transducers of the sending station. When the number of antenna transducers increases, the complexity of such a technique increases rapidly.
What is needed, therefore, is an improved manner by which to implement closed-loop transmit diversity, thereby to permit improved communications in a MIMO, or other, communication system.
It is in light of this background information related to radio communication systems that utilize space diversity that the significant improvements of the present invention have evolved.
The present invention, accordingly, advantageously provides a manner by which to utilize a perturbation gradient approximation technique in the selection of antenna weightings at a sending station that utilizes transmit diversity.
During operation of an embodiment of the present invention, apparatus, and an associated method, is provided by which to select the antenna-weightings at the sending station that utilizes transmit diversity.
Improved antenna-weighting is provided. The procedure automatically adjusts to the rapidity of fading. In slow fading, the short term fading process is tracked. In fast fading, where other methods still unsuccessfully try to track the short term fading, this procedure tracks the long term covariance matrix of the communication channel. The amount of feedback needed for effectuation of the antenna weighting selection is independent of the number of transmit antennas utilized by the sending station. Also, a user-specific pilot signal is not needed for operation. So, the method, and apparatus, is implementable in any of many different types of high data-rate systems, not merely systems that utilize a user-specific pilot signal.
In one aspect of the present invention, a deterministic perturbation gradient approximation procedure is carried out to facilitate the antenna weighting values to be utilized at a sending station that utilizes space diversity. The approximation technique facilitates optimization of the selection of the antenna weighting values, thereby to optimize the communications between a sending station and a receiving station. A perturbation vector is selected at the sending station for communication upon the communication channel to the receiving station. The perturbation vector is selected in a selected order, selected from a selected set of vectors. The vectors are each formed of vector values. The antenna weightings of the antenna transducers of the sending station are perturbed in a first manner during a first portion of a time period and in a second manner during a second portion of a time period. When the time period forms a time slot of a designated time length, during a first half of the time slot, the antennas weightings are perturbed by the perturbation vector in a positive direction. And, during a second half of the time slot, the perturbation vector is applied to the antenna weightings to perturb the weightings in a negative direction.
In another aspect of the present invention, the receiving station that receives the information communicated by the sending station upon the communication channel measures power levels of received signals containing the information communicated by the sending station. Separate power level measurements are made during the first half and the second half of the time slot. Differences between the power levels measured during the separate halves of the time slot are determined. And, an indication of the values of the calculated differences are returned to the sending station.
And, in another aspect of the present invention, the sending station detects delivery of the indications of the values of the calculated differences made at the receiving station and utilizes such indications to adjust the antenna weightings by which subsequently to weight information signals that are sent by the sending station to the receiving station.
In one implementation, the deterministic perturbation gradient approximation technique is utilized in a cellular communication system having a base transceiver station that utilizes space diversity. Closed loop transmit diversity is provided to optimize selection of antenna weightings by which to weight down link signals that are communicated to the mobile station by the base transceiver station. Perturbation vectors are applied to the antenna weighting elements in positive and negative directions during separate portions of a time period. Weighted signals are sent by the base transceiver station to the mobile station and are detected thereat. The mobile station measures power levels of the signals detected thereat during the first and second portions of a time slot and returns values of differences in the power levels measured during the first and second portions of the time slot to the base transceiver station. The values returned to the base transceiver station are used to adjust the antenna weightings at the base transceiver station. Closed-loop transmit diversity is thereby provided. Because deterministic perturbation gradient approximation techniques are utilized, a long-term covariance matrix is tracked and utilized upon which to select the antenna weightings. And, the antenna weighting selection is independent of the number of transmit antennas utilized by the base transceiver station, or other sending station.
In these and other aspects, therefore, apparatus, and an associated method, is provided for a communication system having a first communication station and a second communication station. The first communication station communicates a communication signal to the second communication station. The communication signal is weighted at a first weighting element of the first communication station by a first weighting element of the first communication station by a first antenna weight. And, the communication signal is weighted at least at a second weighting element of the first communication station with at least a second antenna weight. Optimization of selection of the first and at least second antenna weights is facilitated. A perturbation vector selector is coupled to the first communication station. The perturbation vector selector selects at least a first set of perturbation vector values to form at least a first selected perturbation vector. A perturbation vector applicator is coupled to the perturbation vector selector. The perturbation vector applicator applies the perturbation values forming the at least the first selected perturbation vector to the first and at least second weighting elements. The perturbation values applied to the first and at least second weighting elements are at least in part determinative of the first and at least second antenna weights.
A more complete appreciation of the present invention and the scope thereof can be obtained from the accompanying drawings that are briefly summarized below, the following detailed description of the presently-preferred embodiments of the invention, and the appended claims.